Fats and fatty oils, commonly called triglycerides, consist primarily of triesters of glycerol, and include minor amounts of fatty acids. At ambient temperatures, about 20.degree. to about 25.degree. C., fats are solids whereas fatty oils are liquids.
Fats and fatty oils are widely distributed in nature. Many are derived directly from vegetable, animal, and marine sources. Others are obtained, as by-products, in the production of fiber from vegetable matter, and in the production of protein from vegetable, animal or marine matter.
A vast majority of vegetable and animal-derived fats are made up of fatty-acid molecules containing more than about 8 carbon atoms. Marine-derived fats, however, are characterized by their relatively longer-chain fatty acids that may contain up to about 24 carbon atoms.
Throughout this application, the term "oil", and grammatical variations thereof, includes vegetable-derived, animal-derived and marine source-derived fats and fatty oils that are liquids at the particular temperature that is necessary for desired processing of a particular type of oil.
Illustrative sources of edible vegetable oil include canola, coconuts, corn germ, cottonseed, olives, palms, peanuts, rapeseed, safflower, sesame seeds, soybeans, and sunflowers. Examples of nonedible vegetable oils are jojoba oil, linseed oil and castor oil.
Illustrative sources of edible animal-derived oil include lard and tallow. Examples of a nonedible animal-derived oil are low grade tallow and neat's-foot oils.
Some of these oils may have a color that is objectionable to a consumer. Thus, the oil needs to be bleached to improve its color quality. To this end, a great many oils are commonly treated with bleaching clays to reduce oil color values. Bleaching clays generally improve oil color quality by adsorbing color impurities that are present in the oil. Color impurities typically present in oils include, for example, carotenes, xanthophylls, carotenoid acids, xanthophyll esters, chlorophyll, tocopherols and oxidized fatty acids and fatty acid polymers.
It is desirable to remove color impurities from oil not only prior to use but also after use, thereby enabling re-using or recycling the oil. For example, in recent years there has been a substantial growth in the "fast food" type of restaurant. A number of these restaurants specialize in cooking and serving a variety of fried foods which have been prepared in edible cooking oils. Such oils degrade during use and can become noticeably discolored depending upon the nature of the cooking operation, the cooking temperature, and other food-preparing conditions to which such oils have been subjected. The used oil, now discolored, may impart an objectionable color to the food as well. Such color, while not harmful in itself, is often interpreted by the consumer as an indication that the food is substandard or otherwise undesirable. The now discolored oil may therefore be deemed unacceptable for its intended purpose. However, renderers of waste oils commonly remove the color impurities from undesirable oils by bleaching the oil of color impurities to produce a marketable oil.
It is also desirable to remove color impurities from nonedible oil to obtain a desirable color.
Natural clays, e.g., Fuller's earth and bentonite, have commonly been used as bleaching clays to remove both the naturally-occurring and the otherwise-present, e.g., the thermally-induced, color impurities from edible and nonedible oils. It has been suggested that clays containing zeolite can also be used for such a purpose.
Unfortunately, these commercially available natural clays used to remove color impurities from oils remove only the red and yellow color impurities, leaving behind other undesirable color impurities such as chlorophyll. Additionally, these natural clays remove color impurities at the expense of oil filterability. That is, it often becomes necessary to reduce the filtration rate through the clay to achieve such a result. Reduction of oil-filtration rates may involve increased capital expense to maintain current oil-production levels.
An alternative is the use of acid-activated clays. While acid-activated clays remove a wider spectrum of color impurities, their acidity creates other problems such as reduced filter cloth life and the like. These acid-activated clays, which have a pH value of 2 to 5, are more expensive than neutral clays, which have a pH value of about 5 to about 9. Furthermore, acid-activated clays have high residual acid levels which is undesirable. Acid-activated clays may also increase the free fatty acid (FFA) content of the oil.
Bleaching clays are usually not equally efficient at removing color impurities from different oils. Furthermore, there are seasonal variations in the content of color impurities in many vegetable oils. For these reasons, processors of oils must inventory various bleaching clays and select from these a clay which meets the current needs of the processor. Maintaining these inventories is economically undesirable, as is not having a clay suitable for the seasonal content of color impurities.
The spent bleaching clay is saturated with oil and is prone to spontaneous combustion, i.e., the spontaneous development of smoldering sites within the spent clay cake. The onset of smoldering sites is indicated by the development of an acrid odor, then charring of the clay cake, and ultimately fire. This is a common problem that is both undesirable and could be dangerous as well.
The present invention provides a method for the effective bleaching of an oil. Also disclosed is an oil bleaching composition that includes a neutral bleaching clay together with a chelating polycarboxylic acid. This composition also inhibits the spontaneous combustion of the spent bleaching clay. The shortcomings of the aforementioned prior art bleaching methods and compositions are thereby overcome.